Many forms of smoke detectors are known that utilize the so-called Tyndall effect, in which light reflected from smoke particles is detected and the resulting signal amplified to actuate an alarm. Most commercial units utilize a continuously operating incandescent lamp as the light source. Such a detector that has achieved great commercial success is disclosed in U.S. Pat. No. 3,382,762 issed Jan. 25, 1966. Smoke detectors based on this principle have the disadvantage of high current consumption and susceptibility to false alarms due to changing levels of ambient light and changes in line voltage. Hence such devices must be enclosed in a housing that allows diffusion of air into the housing without allowing ambient light to enter, the electrical circuitry must provide means for compensating for changes in line voltage, and the photo-electric detectors must have a high degree of uniformity and stability. Meeting these requirements adds considerably to the cost of the device.
To avoid some of the above disadvantages it has been proposed to utilize a flashing light source, such as a gas filled tube, to reduce the current consumption. It has also been proposed to modulate the pulsed light at a predetermined frequency and provide an amplifier that responds only to said frequency. Such a system is illustrated in U.S. Pat. No. 3,316,410 issued Apr. 25, 1967. it has also been proposed that the means amplifying the signal from the light sensitive element should be operative only while the light source is on, so that ambient light changes or electrical distrubances that occur during the period the amplifier is off cannot cause a false alarm. However, in such a system, ambient light changes and electrical disturbances that occur while the amplifier is on can nevertheless cause a false alarm. Examples of ambient light changes that can affect a detector of this type are flashlights, strong sunlight, turning on of room lights, camera flash bulbs, and lightning. Hence the use of a pulsed light source and a pulsed amplifier as shown in the prior art, although having the advantage of lower power consumption, does little to reduce the possibility of false alarms, and hence to avoid false alarms from such causes the sensitivity of the device must be reduced.